A far infrared region wave is an electromagnetic wave having a frequency range of approximately 0.1 THz to 10 THz. The far infrared region wave has excellent transmittance with respect to many types of materials, such as paper, wood, or plastic, compared to an infrared ray which is an electromagnetic wave having a higher frequency band, and has excellent straightness or resolving power compared to a millimeter wave which is an electromagnetic wave having a lower frequency band.
In addition, an intrinsic absorption spectrum of many types of materials starting from a high molecular compound, such as sugar or protein, is included in a frequency band of a far infrared wave. There is a method of observing a transmitted wave or a reflected wave by irradiating a target material with the far infrared region wave considering the characteristics.
According to the observation method, it is possible to observe an inner structure of the target, the presence or absence of a defect and foreign substances, a difference of materials or intrinsic components, or the like, in a non-destructive manner in a state where the target is in a container having transmittance. Therefore, it is assumed that the observation method can be employed in a material inspection, a structure inspection, or the medicine inspection.
As a generating method of a far infrared region wave of the related art, in 1990s, an optical source and a detector which has a small size, does not require cooling, and uses femtosecond laser were commercialized. At present, a general-purpose spectroscopic measuring device based on time-domain spectroscopy which used the method is commercially available.
Meanwhile, from approximately 2000, a compact coherent optical source of which a frequency is variable in a broadband has been actively studied, and high output characteristics have been developed. Furthermore, a detection technology which used non-linear crystal has also been developed.
In addition, at the same time as emission from high-output Q switch YAG laser, photoinjection (seed optical source) to the non-linear crystal is performed, and accordingly, it is possible to achieve narrowband of generated frequency in far infrared region light. According to the configuration, it is possible to realize both of a broadband and spectroscopic optical source of the narrowband, utilization as a general-purpose spectroscopic optical source widens.